In order to improve the surface properties of metal parts the parts may be coated utilizing thermal spray processes. Thermal spraying involves the use of a thermal spray gun through which a powdered material, typically metal, is propelled at high velocities. As it passes through the spray gun, the powder is heated by combustion gases (flame spraying) or an electric discharge (plasma spraying). The accelerated, high-temperature particles impact the metal target to form a coating which adheres to the target surface. In this manner, the surface properties of a metal part can be significantly altered to suit a particular application.
Through the years, a number of thermal spray powders have been developed. One such class of powders is characterized by composite particles of two or more metals or metal alloys bonded together with or without a binder material. It is also known that these composite powders may consist of a core metal with fine particles of another metal being bonded to the core surface.
For example, in U.S. Pat. No. 4,181,525, a thermal spray powder is described which has particles having a core of nickel, iron, copper, cobalt or alloys thereof coated with a binder. The binder contains discrete particles of aluminum and substantially pure nickel. The core material constitutes from 70-98% of the total mean content of the powder. The core particles range in size between -60 mesh and +3 microns. In addition to aluminum, it is disclosed therein that the binder may further include molybdenum. It has been discovered that although the fine nickel and aluminum help make the coating adherent, machineability is limited by the formation of hard nickel aluminide phases in the coating.
In U.S. Pat. No. 4,578,115, entitled, "Aluminum and Cobalt Coated Thermal Spray Powder," a thermal spray composite is disclosed having a base constituent formed of nickel, iron or cobalt and at east one of the modifying elements, chromium and aluminum, plus, as individual constituents, aluminum, cobalt and, optionally, molybdenum. Each particle comprises an alloy core of the base material and the modifying element, the core having fine particles of the individual elements secured to the core with a binder.
In addition, the manufacture of binderless clad particles by mechanical agglomeration is also known. For example in U.S. Pat. No. 4,915,987, to Nara, et al., a mechanical agglomeration technique is utilized to prepare particles consisting of a core of one material having a cladding of another material. In U.S. Pat. No. 4,818,567 to Kemp, metallic coated particles are disclosed which are formed by preparing a metal flake which is then mechanically applied to the surface of a core particle.
Powders have also been disclosed in which the components react exothermically during spraying. U.S. Pat. No. 3,436,248 entitled "Flame Spraying Exothermically Reacting Intermetallic Compound Forming Composites" describes methods of coating surfaces by flame spraying two or more components which react with one another during flame spraying to form an intermetallic compound. It is stated therein that each particle of the flame spray powder may consist of an aggregate containing the two components which exothermically react, but that preferably the individual particles are in the form of a clad composite consisting of a core of one of the components and at least one coating layer of the other component. It is also disclosed therein that the composite may consist of separate concentric coating layers of the two components and a nucleus of a third material. The methods disclosed for fabricating these prior art powders include chemical plating, vapor deposition, and by dispersing one component in a liquid binder which is then used to coat the core particle. It is stated that the component which is mixed with the binder is finely divided, as for example -325 mesh. It is also disclosed therein that the aggregates may be formed by compacting or briquetting the various components into the individual particles or into larger aggregates and then breaking these aggregates into the granules. The overall particle size is disclosed as between -60 mesh and +3 microns.
Despite the teachings of the prior art, it is clear that a need exists in the industry for a low-cost composite thermal spray powder which is highly adherent and yet which provides excellent machineability. The present invention provides a composite powder which meets these needs.